From fb8e16b2d9211145ba65df748230e76cefecb5b8 Mon Sep 17 00:00:00 2001
From: Haled Odat <8566042+HalidOdat@users.noreply.github.com>
Date: Thu, 28 Mar 2024 00:56:45 +0100
Subject: [PATCH] Cached `this` value (#3771)

---
 core/engine/src/builtins/function/mod.rs      |  8 ++++++-
 core/engine/src/vm/call_frame/mod.rs          |  9 ++++++++
 .../src/vm/opcode/control_flow/return.rs      | 21 ++++++++++++-------
 core/engine/src/vm/opcode/environment/mod.rs  | 12 ++++++++++-
 4 files changed, 40 insertions(+), 10 deletions(-)

diff --git a/core/engine/src/builtins/function/mod.rs b/core/engine/src/builtins/function/mod.rs
index 2be258be07..da223b3bc0 100644
--- a/core/engine/src/builtins/function/mod.rs
+++ b/core/engine/src/builtins/function/mod.rs
@@ -1054,11 +1054,17 @@ fn function_construct(
         Some(this)
     };
 
-    let frame = CallFrame::new(code.clone(), script_or_module, environments, realm)
+    let mut frame = CallFrame::new(code.clone(), script_or_module, environments, realm)
         .with_argument_count(argument_count as u32)
         .with_env_fp(env_fp)
         .with_flags(CallFrameFlags::CONSTRUCT);
 
+    // We push the `this` below so we can mark this function as having the this value
+    // cached if it's initialized.
+    frame
+        .flags
+        .set(CallFrameFlags::THIS_VALUE_CACHED, this.is_some());
+
     let len = context.vm.stack.len();
 
     context.vm.push_frame(frame);
diff --git a/core/engine/src/vm/call_frame/mod.rs b/core/engine/src/vm/call_frame/mod.rs
index e994f4dd00..1913a77f37 100644
--- a/core/engine/src/vm/call_frame/mod.rs
+++ b/core/engine/src/vm/call_frame/mod.rs
@@ -31,6 +31,9 @@ bitflags::bitflags! {
 
         /// Does this [`CallFrame`] need to push registers on [`Vm::push_frame()`].
         const REGISTERS_ALREADY_PUSHED = 0b0000_0100;
+
+        /// If the `this` value has been cached.
+        const THIS_VALUE_CACHED = 0b0000_1000;
     }
 }
 
@@ -323,6 +326,12 @@ impl CallFrame {
         self.flags
             .contains(CallFrameFlags::REGISTERS_ALREADY_PUSHED)
     }
+    /// Does this [`CallFrame`] have a cached `this` value.
+    ///
+    /// The cached value is placed in the [`CallFrame::THIS_POSITION`] position.
+    pub(crate) fn has_this_value_cached(&self) -> bool {
+        self.flags.contains(CallFrameFlags::THIS_VALUE_CACHED)
+    }
 }
 
 /// ---- `CallFrame` stack methods ----
diff --git a/core/engine/src/vm/opcode/control_flow/return.rs b/core/engine/src/vm/opcode/control_flow/return.rs
index 2296c41021..31e4294e0f 100644
--- a/core/engine/src/vm/opcode/control_flow/return.rs
+++ b/core/engine/src/vm/opcode/control_flow/return.rs
@@ -54,16 +54,21 @@ impl Operation for CheckReturn {
             );
             return Ok(CompletionType::Throw);
         } else {
-            let realm = context.vm.frame().realm.clone();
-            let this = context.vm.environments.get_this_binding();
+            let frame = context.vm.frame();
+            if frame.has_this_value_cached() {
+                this
+            } else {
+                let realm = frame.realm.clone();
+                let this = context.vm.environments.get_this_binding();
 
-            match this {
-                Err(err) => {
-                    let err = err.inject_realm(realm);
-                    context.vm.pending_exception = Some(err);
-                    return Ok(CompletionType::Throw);
+                match this {
+                    Err(err) => {
+                        let err = err.inject_realm(realm);
+                        context.vm.pending_exception = Some(err);
+                        return Ok(CompletionType::Throw);
+                    }
+                    Ok(this) => this,
                 }
-                Ok(this) => this,
             }
         };
 
diff --git a/core/engine/src/vm/opcode/environment/mod.rs b/core/engine/src/vm/opcode/environment/mod.rs
index 0f3dcb4b9c..2d04092ec7 100644
--- a/core/engine/src/vm/opcode/environment/mod.rs
+++ b/core/engine/src/vm/opcode/environment/mod.rs
@@ -2,7 +2,7 @@ use crate::{
     builtins::function::OrdinaryFunction,
     error::JsNativeError,
     object::internal_methods::InternalMethodContext,
-    vm::{opcode::Operation, CompletionType},
+    vm::{opcode::Operation, CallFrameFlags, CompletionType},
     Context, JsResult, JsValue,
 };
 
@@ -19,7 +19,17 @@ impl Operation for This {
     const COST: u8 = 1;
 
     fn execute(context: &mut Context) -> JsResult<CompletionType> {
+        let frame = context.vm.frame_mut();
+        let this_index = frame.fp();
+        if frame.has_this_value_cached() {
+            let this = context.vm.stack[this_index as usize].clone();
+            context.vm.push(this);
+            return Ok(CompletionType::Normal);
+        }
+
         let this = context.vm.environments.get_this_binding()?;
+        context.vm.frame_mut().flags |= CallFrameFlags::THIS_VALUE_CACHED;
+        context.vm.stack[this_index as usize] = this.clone();
         context.vm.push(this);
         Ok(CompletionType::Normal)
     }